Both the UK and Scottish governments have been criticised by MPs probing whether civil servants were impartial during the referendum campaign.

A Westminster committee said advice from the most senior UK Treasury civil servant on a currency union should not have been published.

It also questioned the use of public money for the Scottish government's independence White Paper.

The MPs said guidance to civil servants should now be changed.

In February of last year Chancellor George Osborne released a report from Sir Nicholas MacPherson, the permanent secretary at the Treasury, which said currency unions were "fraught with difficulty".

Sir Nicholas "strongly" advised against a currency union with an independent Scotland "as currently proposed" by the Scottish government.

The SNP complained about a lack of impartiality from Sir Nicholas.

'Political objectives'

A report by the House of Commons Public Administration Committee has now said the advice should have remained confidential.

The letter from Sir Nicholas was only made public "because it suited ministers' political objectives in respect of the Scottish referendum", the committee said.

The MPs added that this had "compromised the perceived impartiality of one of the UK's most senior civil servants".

Sir Nicholas told a lecture to the Strand Group earlier this year he had allowed the advice to be published "because I regarded it as my duty".

He said: "The British state's position was being impugned. Demonstrating that the political and official state were completely aligned would further strengthen the credibility of the government's position. And it was important that the arguments were exposed before the referendum rather than after it."

Scotland's former first minister and SNP leader Alex Salmond told the BBC's Andrew Marr programme on Sunday that Sir Nicholas's behaviour had been "unprecedented".

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The Scottish government rejected the committee's criticisms of the independence White Paper

Mr Salmond said: "I think when you have somebody who is prepared to enter the political arena as a civil servant in that fashion then you are in a situation where not only a long-standing mistrust of the Westminster Treasury but a mistrust of the personnel involved is very evident."

A spokesman for HM Treasury said: "As we have made clear before, the question of whether or not the UK would agree to a currency union was an exceptional case where it was important that the arguments were exposed in full before a referendum rather than after it."

The Commons committee also concluded the Scottish government's independence White Paper had "raised questions about the use of public money for partisan purposes", as part of the 670-page long blueprint that set out SNP pledges ahead of the 2016 Holyrood election.

Its report stated that "parts of the white paper should not have been included in a government publication".

It also said civil servants "should always advise against the appearance of partisan bias in government documents" and they should "not be required to carry out ministers' wishes, if they are being asked to use public funds to promote the agenda of a political party, as was evident in this case".

In response, a Scottish government spokeswoman said the document "met the highest professional standards, that its contents were entirely appropriate for a government publication and was a proper use of public funds".

'Exceptional circumstances'

She added: "The Scottish and UK governments have frequently set out policy intentions whose implementation depends on the outcome of future elections.

"Indeed, just last week, the UK government budget set out a range of policies of the governing parties where implementation will fall beyond the general election."

The Public Administration Committee said lessons must be learned from the Scottish experience so future uncertainty about the role of the civil service could be avoided.

It called for the Civil Service Code to be "revised to specifically refer to referendums and provide civil servants across the UK with clear and definitive guidance on their role in respect of referendum campaigns".

Committee chairman and Conservative MP Bernard Jenkin said: "The Scottish independence referendum created exceptional circumstances, but that does not make it acceptable for parts of the civil service to approve the use of public funds to promote the agenda of one political party, to become personally aligned with one side or the other in the referendum debate."